Theoretical works on potentiometric titration of ion exchangers are reviewed. Most attention is paid to the approach considering this process as a specific case of ion exchange between H+ or OH- and the counterions of the titrant, Mz+ or A(z-), accompanied by the neutralization reaction. An equation connecting the most important properties of the ion exchanger-solution system (pH, concentration of the electrolyte in the solution phase and degree of neutralization of the ion exchanger) named ＇acidity function＇ is discussed in detail. Practical methods for determination and use of a set of its three constants, sufficient for calculation of ionic equilibria in ion-exchange systems (parameters of acidic-basic strength of ion exchangers), are described and illustrated with experimental examples. The acidity function is used for computer simulation of potentiomertic titration curves of ion exchangers to observe the influence of different factors on position and shape of the potentiometric titration curves and chose the best possible conditions for observing the inflection points. The physical meaning of the constants of the modified Henderson-Hasselbach equation are discussed and their relations with the parameters of acidic-basic strength of ion exchangers are expressed in the quantitative form.